Field of the Invention and Related Art Statement
This invention relates to an exhaust gas treatment apparatus and method which can effectively remove sulfur and ammonia from an exhaust gas in a small-sized apparatus constitution, particularly to an apparatus and a method for exhaust gas treatment which can attain a high ammonia removal ratio.
As such a type of an exhaust gas treatment method, conventionally known is a method for removing sulfur oxides (mainly a sulfur dioxide gas) and the like from an exhaust gas by subjecting an absorbing liquid having an absorber such as limestone suspended therein and the exhaust gas to vapor-liquid contact by using a packed absorption tower (a vapor-liquid contact tower) or a spray type or liquid-column type absorption tower.
In general, the conventional packed-type exhaust gas treatment apparatus or spray type or liquid-column type exhaust gas treatment apparatus is essentially formed of one vapor-liquid contact tower per absorbing liquid tank. Such a constitution however has imposed a limitation to the size reduction, cost reduction and maintainability improvement as well as improvement in the performance of desulfurization.
Described specifically, in order to heighten the performance, an increase in the row number of spray nozzles to be installed in the case of the spray type, an increase in the height of the liquid column in the case of the liquid-column type and an increase in the height of the packed portion in the case of the packed type are required, which causes a marked increase in the whole size of the apparatus (particularly, the height of the absorption tower), the number of ducts or pipes connected with the apparatus or an installation height, resulting in the considerable increase in the capacity or power of a pump for pumping up the absorbing liquid.
The present inventor has therefore proposed in Japanese Patent Application No. HEI 5-118171 (Japanese Patent Application Laid-Open No. HEI 6-327927) an apparatus which exceeds the limitation of the conventional apparatus and has improved performance and reduced size.
The proposed apparatus is a so-called parallel counter flow type in which two liquid-column type absorption towers (parallel flow type and counter flow type) are aligned above a tank for storing an absorbing liquid and an exhaust gas is introduced into each absorption tower successively, where vapor-liquid contact of the exhaust gas with an absorbing liquid is effected in each tower. Such a constitution makes it possible to reduce the whole size (mainly a reduction in the height of the absorption tower) and to reduce the cost (reduction in both the equipment cost and operation cost) and moreover to improve the desulfurization and dedusting performance.
A description will next be made of one example of a conventional exhaust gas treatment method using such a parallel-counter flow type vapor-liquid contact apparatus, with reference to FIG. 3.
In this exhaust gas treatment method, employed is a vapor-liquid contact apparatus formed of a tank 1 to which an absorbing liquid (which will hereinafter be called "absorbent slurry") having an absorbent containing limestone suspended therein is fed; a liquid-column type introduction-side absorption tower 2 which extends upward from one side portion of the tank 1 and brings an untreated exhaust gas A into vapor-liquid contact with the absorbent slurry in the tank 1; and a liquid-column type ejection-side absorption tower 3 which extends upward from the other side portion of the tank 1 and brings the exhaust gas from the introduction-side absorption tower 2 into vapor-liquid contact with the absorbent slurry in the tank 1 again.
Here, the introduction-side absorption tower 2 is a so-called parallel flow type absorption tower in which an untreated exhaust gas A flows downward, introduced from the upper part. The ejection side absorption tower 3 is a so-called counter flow type absorption tower in which an exhaust gas ejection portion (not illustrated) for ejecting the treated exhaust gas B is formed at the upper part of the tower and the exhaust gas passing through the introduction-side absorption tower 2 and then the upper part of the tank 1 flows upward.
In the absorption towers 2,3, spray pipes 4,5, each in plural, are arranged in parallel, respectively. Each of these spray pipes 4,5 has plural nozzles (not illustrated) for injecting the absorbent slurry upward in the form of a liquid column.
Disposed outside the tank 1 are circulation pumps 6,7 for sucking the absorbent slurry from the tank 1. The absorbent slurry is fed into spray pipes 4,5 through circulation lines 8,9, respectively and then, injected upward from each nozzle.
The apparatus as shown in FIG. 3 is equipped with a so-called arm rotating type air sparger 10 for blowing oxidation air C in the form of fine air bubbles into the tank 1 while stirring the slurry in the tank 1, whereby the absorbent slurry which has absorbed a sulfur dioxide gas is efficiently brought into contact with air to cause complete oxidation, thereby affording gypsum.
According to this method, the absorbent slurry, which is brought into vapor-liquid contact with the exhaust gas after being injected from the header pipe 4 or 5 in the absorption tower 2 or 3 and flows downward while absorbing a sulfur dioxide gas and dust, is oxidized by being brought into contact with many air bubbles blown and stirred by the air sparger 10 in the tank 1, and then causes neutralization reaction, whereby a slurry containing gypsum at a high concentration is obtained. Incidentally, the main reactions occurring during the above treatment can be indicated by the following reaction formulas (1) to (3).
(Exhaust gas introduction portion of absorption tower) EQU SO.sub.2 +H.sub.2 O.fwdarw.H.sup.+ +HSO.sub.3.sup.- (1)
(tank) EQU H.sup.+ +HSO.sub.3.sup.- +1/2O.sub.2 .fwdarw.2H.sup.+ +SO.sub.4.sup.2-(2) EQU 2H.sup.+ +SO.sub.4.sup.2- +CaCO.sub.3 +H.sub.2 O.fwdarw.CaSO.sub.4 .multidot.2H.sub.2 O+CO.sub.2 (3)
In the tank 1, a large amount of gypsum, a small amount of limestone as an absorbent and a slight amount of powdery dust collected from the exhaust gas regularly exist in the form of a suspension or solution. This slurry in the tank 1 is fed to a solid-liquid separator 11 through a pipe line 9a branched from the circulation line 9 and is then collected by filtration as gypsum D having a small water content. The filtrate from the solid liquid separator 11, on the other hand, passes through a filtrate tank 12 and then pumped out by a pump 13. A portion of the filtrate is returned to the tank 1 as a water content to form an absorbent slurry, while another portion is discharged as a desulfurized waste water E in order to prevent the accumulation of impurities.
To the tank 1 under operation, limestone which is an adsorbent is supplied from a slurry preparation tank 15 as a slurry. The slurry preparation tank 15 has an agitator 16 and there, powdery limestone F charged from a silo (not illustrated) and water G supplied (industrial water or the like) are stirred and mixed, whereby an absorbent in the form of a slurry is prepared. The absorbent so prepared is then supplied by a slurry pump 17 to the tank 1 as needed.
In addition, make-up water (industrial water or the like) is supplied to the tank 1 to supplement the water content which shows a gradual decrease by the evaporation in the absorption towers 2,3.
During operation, the flow rate of the make-up water to the tank 1 or the flow rate of the slurry ejected from the pipe line 9a is controlled, whereby a slurry containing predetermined concentrations of gypsum and adsorbent is always stored in the tank 1 in an amount within a predetermined range.
During operation, a boiler load (flow rate of the exhaust gas A), a sulfur dioxide gas concentration in the untreated exhaust gas A, and pH and limestone concentration in the tank 1 are detected by sensors with a view to maintaining the desulfurization ratio and gypsum purity at a high level. Based on these detection results, a supply rate of the limestone to the tank 1 is adjusted by a controller (not illustrated) as needed. In general, the pH in the tank 1 is conventionally adjusted to 6.0 or so in order to promote the above-described oxidation reaction, while maintaining the high absorption performance of the sulfur dioxide gas, thereby preparing high-purity gypsum.
The exhaust gas treatment method using the above-described vapor-liquid contact apparatus of a parallel counter flow type which method has been proposed and industrialized by the applicant, however, is accompanied with the problem that when the untreated exhaust gas A contains ammonia, this ammonia is released as a gas in the counterflow type ejection-side absorption tower 3 and is then inevitably discharged while being contained in the treated exhaust gas B.
For example, an exhaust gas treatment equipment for an oil burning boiler of a thermal electric power plant, sulfur trioxide (SO.sub.3) contained in the exhaust gas is collected as ammonium sulfate ((NH.sub.4).sub.2 SO.sub.4) so that ammonia is generally poured into the exhaust gas at the flow prior to a desulfurizing apparatus, and an untreated exhaust gas A to be introduced to the desulfurizing absorption tower therefore contains about 100 ppm of ammonia at the maximum. In the conventional parallel counter flow type vapor-liquid contact apparatus as illustrated in FIG. 3, a large portion of the ammonia is dissolved and absorbed in the slurry in the parallel flow type introduction-side absorption tower 2 so that the ammonia concentration in the exhaust gas lowers to about 10 ppm at the maximum at the outlet portion of the introduction-side absorption tower 2. The slurry injected to the upper part of the ejection-side absorption tower 3 and brought into contact with the exhaust gas has pH as high as about 6, which increases the partial pressure of ammonia at the upper part of the absorption tower 3. Ammonia in the slurry is released again to the gas side, resulting in an increase in the ammonia concentration in the treated exhaust gas B to about 50 ppm at the maximum.
Incidentally, the exhaust gas control in Japan has not applied to ammonia yet, but it is desired to minimize the ammonia concentration in the treated exhaust gas B to be released into the air from the viewpoint of the air pollution prevention. There is accordingly a demand for the exhaust gas treatment method which can accomplish the size reduction of the apparatus and high desulfurization performance and at the same time reduce the amount of discharged ammonia.